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Tuesday, December 15, 2015

Gamma Rays from Distant Galaxy Tell Story of an Escape

A flare of very high-energy gamma rays emitted from a galaxy halfway across the universe has put new bounds on the amount of background light in the universe and given astrophysicists clues to how and where such gamma rays are produced. The galaxy, known as PKS 1441+25, is a rare type of galaxy called a blazar, a tremendously bright beacon powered by a supermassive black hole at the heart of the galaxy. Blazars are intrinsically unsteady light sources and can sometimes emit flares ten to a hundred times brighter than their baseline emissions. A flare from PKS 1441+25 was detected in April 2015 and observed by a range of telescopes sensitive to different wavelengths, including the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona.

"With VERITAS, we detected gamma rays from this unusual object at the highest energies observed on Earth," said Jonathan Biteau, who led the analysis of the data as a postdoctoral researcher at UC Santa Cruz.

Such very high-energy gamma rays were unexpected, he said, because they faced a good chance of being annihilated at some point during the 7.6 billion years they spent traveling toward Earth. When high-energy gamma rays collide with lower-energy photons, they annihilate and create an electron-positron pair. To reach telescopes on Earth, the gamma rays from PKS 1441+25 had to avoid a tight net of photons surrounding the vicinity of the black hole, as well as a looser net of photons, the extragalactic background light (EBL), that fills the universe.

"We're looking down the barrel of this relativistic jet," explains Wystan Benbow of the Harvard-Smithsonian Center for Astrophysics (CfA). "That's why we're able to see the gamma rays at all."

The EBL is a faint glow that pervades the space between galaxies, consisting of photons from all the stars and galaxies that have existed. It is hard to measure because there are so many bright sources of light nearby. Astronomers have used cosmological models to estimate the EBL, galaxy counts to set lower limits, and gamma rays from blazars (as well as direct observations of dark patches of sky) to set upper limits. The farther gamma rays have to travel the more likely they are to encounter photons of the EBL and annihilate, so the detection of a source 7.6 billion light-years away was surprising.

"With PKS 1441+25, we can now place tight constraints on this loose net of photons," said Biteau. "This is clearly the opening of a new era where we can compare source-by-source measurements and start to probe the cosmic evolution of the extragalactic background light."

Biteau, now an associate professor at Institut de Physique Nucléaire d'Orsay in France, and UC Santa Cruz graduate student Caitlin Johnson are corresponding authors of a paper on the findings to be published inAstrophysical Journal Letters and currently available online. Johnson analyzed data from the Fermi Gamma-ray Space Telescope which complemented the data from VERITAS.

"Combining the Fermi data with the VERITAS data enabled us to make the constraints on the EBL much tighter," she said. "The window is slowly narrowing."

But what about the tight net of photons around the tremendously bright blazar? That helps locate the region where the gamma rays were emitted, said coauthor David Williams, adjunct professor of physics at UC Santa Cruz. "If the gamma rays were produced close to the black hole, the radiation fields there are strong enough to absorb them. So the fact that the gamma rays are getting out of the galaxy at all indicates they were produced farther away from the black hole," Williams said.

The black hole is surrounded by a glowing disk of hot gas and dust swirling in toward the center. Some of this infalling matter, instead of being swallowed by the black hole, gets channeled into two powerful jets emitted from the poles of the spinning black hole, perpendicular to the accretion disk. One of these jets is pointing in our direction, "like a flashlight shining in our eyes," Johnson said.

Physicists are still debating the exact mechanism behind gamma-ray emissions from the jets, but PKS 1441+25 provides important clues, Biteau said. "With observations across the entire electromagnetic spectrum, we have now realized that the location of the gamma-ray emissions for this source has to be at least a tenth of a light year away from the black hole. Otherwise, none of the gamma rays would escape," he said.

The researchers estimated that the emission region is probably at a distance of about five light years from the black hole, much further than expected. According to a leading scenario for the gamma-ray emissions, high energy electrons are accelerated to near the speed of light in the jet, interact with photons, and transfer their energy, boosting the photons to gamma-ray energies.

In April, PKS 1441+25 underwent a major eruption. Luigi Pacciani at the Italian National Institute for Astrophysics in Rome was leading a project to catch blazar flares in their earliest stages in collaboration with the Major Atmospheric Gamma-ray Imaging Cerenkov experiment (MAGIC), located on La Palma in the Canary Islands. Using public Fermi data, Pacciani discovered the outburst and immediately alerted the astronomical community. Fermi's Large Area Telescope revealed gamma rays up to 33 billion electron volts (GeV), reaching into the highest-energy part of the instrument's detection range. For comparison, visible light has energies between about 2 and 3 electron volts.

"Detecting these very energetic gamma rays with Fermi, as well as seeing flaring at optical and X-ray energies with NASA's Swift satellite, made it clear that PKS 1441+25 had become a good target for MAGIC," Pacciani said.

Following up on the Fermi alert, the MAGIC team turned to the blazar and detected gamma rays with energies ranging from 40 to 250 GeV. "Because this galaxy is so far away, we didn't have a strong expectation of detecting gamma rays with energies this high," said Josefa Becerra Gonzalez, a researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who analyzed Fermi LAT data as part of the MAGIC study. "There are fewer and fewer gamma rays at progressively higher energies, and fewer still from very distant sources."

PKS 1441+25 is one of only two such distant sources for which gamma rays with energies above 100 GeV have been observed. Its dramatic flare provides a powerful glimpse into the intensity of the EBL from near-infrared to near-ultraviolet wavelengths and suggests that galaxy surveys have identified most of the sources responsible for it.

In addition to Biteau, Johnson, and Williams, the coauthors of the study include an international team of researchers from 30 institutions in the United States, Canada, Ireland, France, and Germany. This research is funded by the U.S. Department of Energy, National Science Foundation, Smithsonian Institution, NASA, and by NSERC in Canada.